Apparatus for converting side-to-side driving motion to rotational motion with a spring assembly and system for tuning the spring assembly

ABSTRACT

A method for tuning resonant spring assembly systems used in appliances such as power toothbrushes, particularly leaf spring systems, includes the step of measuring the frequency-dependent amplitude of the appliance workpiece, and then removing a portion of the spring member so as to decrease its spring rate sufficiently that the resonant frequency of the spring member closely approximates the drive frequency of the appliance.

PRIOR APPLICATION

This is a continuation of U.S. patent application Ser. No. 11/397,072Apr. 3, 2006 now abandoned, the priority of which is hereby claimed.

TECHNICAL FIELD

This invention relates generally to small appliances, such as, forinstance, toothbrushes, and more specifically concerns a springstructure for converting a side-to-side driving action into a rotaryworkpiece action. The invention also concerns a method for tuning thespring structure to a selected natural resonant frequency or amplitudeduring manufacture.

BACKGROUND OF THE INVENTION

Small appliances have various workpiece motions. In some cases, thedrive motion is the same as the workpiece motion; in other cases, it isdesirable to convert a particular drive motion into a differentworkpiece motion, such as a side-to-side drive motion to a rotarymotion. The magnetic driver shown in U.S. Pat. No. 5,189,751 moves oneend of an elongated pivoted arm to which is attached a toothbrushelement, in a side-to-side (back and forth), slightly arcuate manner.The workpiece vibrates in a particular manner accordingly. While thatworkpiece motion does provide good results, it may be desirable, forvarious reasons, to have the workpiece rotate through a selected arc(rotational motion), while retaining the side-to-side driving action.This would require, however, a motion conversion assembly, fromside-to-side to rotational.

Apparatus for converting a side-by-side driving action, such as producedby the electromagnetic driver of the '751 patent, to a workpiece rotaryaction are known. Some devices involve linkage-type arrangements.However, such linkage arrangements frequently have undesirable backlashaction, which can cause damping, vibration and noise. Pivot assembliesare also used, in combination with a spring element. Torsion springs canbe used, but they are usually made from coil-type springs, whichtypically do not combine the required radial stiffness with anacceptably low torsion spring rate. Most of these known devices requirevarious bearing structures, which are often complex, noisy andunreliable. Bearings also have their own backlash conditions.

Further, in another aspect of the present invention, spring elements areoften used as part of a driving assembly in a resonant system, such asthe spring assembly of the present invention. These spring assembliesusually require either tuning in some fashion or very closemanufacturing tolerances, so that the resonant frequency of the springassembly is acceptably close to the operating or drive frequency of thedevice, to maintain the efficiency of the resonant system.

With respect to the motion conversion assembly aspect of the presentinvention, it is desirable that the conversion assembly be efficient,reliable and inexpensive to manufacture. With respect to the springassembly tuning, it is desirable that the tuning be accomplished with asimple method which can be carried out during manufacture of theassembly so that the manufacturing tolerances of the appliance can beincreased (greater tolerances), thereby decreasing the cost ofmanufacture of the appliance, as well as substantially reducing thenumber of appliances rejected during manufacture.

SUMMARY OF THE INVENTION

Accordingly, one aspect of the present invention is an apparatus andcorresponding method of converting side-to-side drive motion torotational motion for a workpiece portion of an appliance, comprising: amovable base mounting element, capable of being driven in atranslational reciprocal motion; a fixed position mounting elementspaced apart from the base element, the fixed mounting element beingheld so that it does not move during operation of the apparatus; aspring member connected to the base element and to the fixed mountingelement; and a drive shaft member connected to the base element androtatable relative to the fixed mounting element, the drive shaft havinga workpiece mounted thereto, wherein the spring member is so configuredand arranged that it is substantially less resistant to twisting actionthan to bending action, the twisting action producing workpiece rotationat a preselected frequency.

Another aspect of the present invention is a method for tuning a springmember used in a resonant driving system for an appliance which includesa workpiece and which has a driving frequency, comprising the steps of:measuring the frequency dependent amplitude of the workpiece whichvibrates by action of the spring member, wherein the measured frequencyis either above or below a driving frequency of the appliance; andchanging the spring rate of the member, and hence its resonantfrequency, sufficiently that the resonant frequency of the spring memberclosely approximates the driving frequency of the appliance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A are exploded views of the motion conversion assembly ofthe present invention shown in the context of a power toothbrush.

FIG. 2 is a perspective view of the motion conversion assembly portionof FIG. 1.

FIG. 3 is an elevational view showing the angled orientation of thedrive shaft and the brush arm relative to the longitudinal axis of thetoothbrush.

FIG. 4 shows a modified motion conversion system used for a shavingappliance.

FIG. 5 shows the result of the frequency tuning method of the presentinvention on the motion conversion assembly of FIG. 2.

FIGS. 6-8 show one alternative embodiment to the invention of FIG. 2.

FIGS. 9-10 show another alternative embodiment of the invention of FIG.2.

FIG. 11 shows still another alternative embodiment to the invention ofFIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 1A show a power toothbrush 10 which includes a handleportion 12 and a head portion 14. The handle portion includes a powersource, such as a rechargeable battery 16, and a drive assembly, showngenerally at 18. The head portion 14 includes a workpiece element, e.g.a brushhead 20, which comprises a plurality of bristles arranged in aselected pattern, a brushhead arm 22 on which brushhead 10 is mounted, amotion conversion assembly, shown generally at 26, and a mountingassembly 27 for the motion conversion assembly.

The motion conversion assembly 26 in the embodiment shown converts alinear action to rotation of a drive shaft 28 which extends into andjoins with brushhead arm 22 for rotation thereof and rotation ofbrushhead 20. Head portion 14 also includes a nut element 30, whichconnects head portion 14 to handle portion 12 and to which the mountingassembly 27 is fixedly secured.

In the appliance shown, the driving assembly 18 is an electromagnet,which produces a side-to-side force and cooperates in operation with twopermanent magnets 32 mounted to a movable end piece 40 at the rear endof the motion conversion assembly 26 to move end piece 40 in aside-to-side slightly arcuate, translational manner. “Side-to-side”herein refers to a straight side-to-side motion or a side-to-side pathwhich is slightly arcuate. The motion conversion assembly 26 convertsthe drive action of the drive assembly via a leaf spring arrangementinto a twisting or rotational action of drive shaft 28, which in turnrotates brushhead arm 22 and brushhead 20 about the longitudinal axisA-A of drive shaft 28 (FIG. 2). In the embodiment shown, the includedangle (arc) of rotation of the brushhead is approximately 11°, althoughthis angle can be varied and is not an essential part of the presentinvention.

The motion conversion assembly 26 is shown in more detail in FIG. 2. Itincludes movable end piece 40, which in the embodiment shown is madefrom a plastic material and is approximately 0.6 inches long,approximately 0.6 inches wide at its widest dimension, and approximately0.1 inches thick. Extending from the rear face 44 of end piece 40 is asmall, centrally located mounting stub 46. Mounted on stub 46 is apermanent magnet assembly, which in the embodiment shown comprises ametal mounting plate 50 (FIG. 1) and two spaced rectangular permanentmagnets 32-32. Permanent magnets 32-32 interact with electromagnet 18 inhandle 12 such that the movable end piece 40 moves from side-to-side ina slightly arcuate path. This action is explained in more detail in the'751 patent, the contents of which are hereby incorporated herein byreference. However, it should be understood that the side-to-side driveaction by the drive assembly 18 is only one such arrangement. Many otherside-to-side drivers are known and can be used with the conversionassembly of the present invention.

On the opposing face 56 of movable end piece 40 is a mounting piece 58which is either integral with end piece 40 or is securely attachedthereto. Extending forwardly from mounting piece 58 are two elongatedleaf springs 60 and 62. In the embodiment shown, each leaf spring 60, 62is approximately 1 inch long, 0.2 inches wide and approximately 0.02inches thick. Leaf springs 60, 62 in the embodiment shown are made frommetal. However, they could be made from other materials, such asplastic.

The two leaf springs 60, 62 in the embodiment shown are positioned at anangle of 70° relative to each other. The two leaf springs extend betweenand are fixedly mounted to both movable end piece 40 and fixed end piece64. Fixed end piece 64 is generally circular, approximately 0.1 inchesthick, and in the embodiment shown is made from plastic. It has twoopposed notch openings 67, 69 at the top and bottom portions thereof.

Extending from mounting piece 58 approximately at what would be theintersection of the two leaf springs 60 and 62, if the leaf springs wereso extended, is the elongated drive shaft 28. The planes of the twosprings 60 and 62 intersect at the center of rotation of the motionconversion assembly, where the drive shaft is located. In the embodimentshown, drive shaft 28 is metal and is rectangular in cross-section. Thedrive shaft 28, however, could have other configurations. In operation,the leaf springs twist and also bend somewhat as well, producing arotation of the drive shaft. The relative position and dimensions of thesprings can be optimized to reduce stress in each spring independently.

Drive shaft 28 in the embodiment shown extends through notched opening69 in the fixed end piece 64, so that it is free to rotate relative tofixed end piece 64. Drive shaft 28 extends to and is joined to brushheadarm 22, so that rotation of drive shaft 28 produced by the action of themotion conversion assembly produces a rotation of brushhead arm 22 andbrushhead 20 mounted at the distal end thereof.

The fixed end piece 64 is secured by mounting assembly 27 so that itdoes not rotate relative thereto. The mounting assembly 27 includes as aportion thereof a ring of plastic material 72, with two opposing flanges74 and 76 which extend rearwardly therefrom. The two flanges 74 and 76extend through the notched openings 67, 69 in the fixed end piece 64.The two flanges 74 and 76 are configured to mate with correspondingreceiving portions (not shown) in handle 12 of the toothbrush to producea secure mating connection between the mounting assembly 27 (and headportion 14) and handle 12. When flanges 74 and 76 of the mountingassembly 27 are properly positioned in the receiving portions of handle12 and nut 30 is in place, as described below, the mounting assembly 27and, hence, the fixed end piece 64 are held firmly from, i.e. preventedfrom, any rotational action. The distal ends of the two leaf springs 60and 62 are fixedly mounted in end piece 64 and also hence prevented frommoving.

Fitting over the mounting assembly 27 and the motion conversion assembly26 is connecting nut element 30. The connecting nut element has threads77 on its internal surface so that it can be screwed onto an externalthreaded portion 67 of the handle 12. The nut 30 clamps flanges 74 and76 to the handle, with fixed end piece 64 being held in place by flanges74 and 76. At the upper edge 79 of nut 30 is a flexible connectingmember 78, which in the embodiment shown is made from an elastomericmaterial. The lower edge of connecting member 78 fits flush with upperedge 79 of nut 30. Member 78 extends to a seal element 77, whichprovides a fluid seal between brushhead arm 22 and the upper end ofconnecting member 78.

In operation, the side-to-side drive produced by the electromagneticdriver 18 in the handle (or any other side-to-side driver) produces aslightly arcuate action of movable end piece 40, resulting in a rotationof the brushhead 20 through a specific angle.

In the embodiment shown, the two leaf springs 60 and 62 and the driveshaft 28 are mounted in the fixed and movable end pieces such that theyextend at an angle α away from the longitudinal axis B-B of thetoothbrush (FIG. 3). This angle is within the range of 5-15° andpreferably 10°. This angle of the brushhead arm provides an increasedreach for the toothbrush into the user's mouth. This is an advantage inmany instances. Further, with this arrangement, the center of rotationis on a different axis than the axis of the handle. As the center ofrotation is tilted away from the longitudinal axis of the appliance, themoment of inertia of the rest of the appliance relative to the axis ofrotation of the drive shaft increases rapidly, thereby reducing thevibration of the handle, since the greater the moment of inertia, theless resulting vibration in the appliance, including the handle. This isa desirable advantage for the user.

While the embodiment described includes two separate leaf springs whichare at a specific angle (70°) to each other, two separate leaf springsare not necessary. The separation angle could vary within a range of90°±40%. In addition, more than two leaf springs could be used, in aradial pattern, with the plane of the leaf springs intersecting thecenter of rotation of the drive shaft. The two leaf springs could bejoined into a single leaf spring in a “V” or a “U” form, an open squareor a rectangle. Other configurations are also possible. The drive shaftmust be connected to the movable end piece portion of the system, withthe springs. The drive shaft cannot be connected to any other part ofthe springs. This structure could be a single part. The key structuralrequirement is that the spring element must be configured so that it issubstantially less resistant to twisting than to bending, i.e. so thatthe side-to-side action of the movable end piece results in a twistingof the springs and a resulting rotation of the drive shaft. Thearrangement shown and described herein produces such an action. Otherspring arrangements could be used, however. Further, and alsoimportantly, the arrangement of the springs must be such that thebrushhead will not only rotate, but rotate at a selected frequency.

FIGS. 6, 7 and 8 show one alternative embodiment to the motionconversion assembly of FIGS. 1-3 using a particular arrangement of awire form spring. The conversion assembly includes a back plate 120 andpermanent magnets 122. An armature shaft 124 is mounted to back iron 122on stub 126. Shaft 124 has two slots 123, 125 at 90° (orthogonal) toeach other, in the distal end 127 thereof. Two wire form springs 128 and130, also positioned at 90° (orthogonal) to each other, are insertedinto the slots 123, 125. A brush shaft 129 has cross elements 131, 133which mate with slots 123, 125, capturing the wire form springs betweenthe respective ends of the armature shaft 124 and the brush shaft 129.

At the distal end 135 of the brush shaft is a base element 137 for abrushhead or other workpiece (not shown). A collar 136 is pressed downthe brush shaft onto the armature shaft, holding the springs, the brushshaft and the armature shaft together, like a collet. The rear ends ofwire form springs 128 and 130 are then captured within a spring mount138, which is slotted 141 at its rear end 139 thereof to capture theends of the wire form springs. The spring mount 138 is fixedly connectedto the housing of the appliance. Movement of the back plate 120, causedby side-to-side force produced by the drive assembly, results inrotation of brush shaft 129.

FIGS. 9 and 10 show a further embodiment. The conversion assemblyincludes a single-piece, multi-leaf spring 140, comprising threeseparate legs 142-144 joined together at one end 148. The spring 140 ismounted to a movable rear member 150, in which is positioned a tuningmass 151 from which material can be removed to tune the resonantfrequency of the spring assembly. A back plate element 152 is positionedon the rear surface of the movable rear member 150, to which permanentmagnets 154 are secured. A drive shaft 158 is attached to and extendsfrom the movable member 150. To the distal end 155 of drive shaft 158 issecured a workpiece, such as a brushhead (not shown). The free ends ofthe spring assembly 140 are secured to a fixed mounting element 160,through which the drive shaft 158 extends. The mounting element 160 issecured to the housing of the toothbrush. Reciprocating motion of themoving member 150 results in a rotational motion of the drive shaft 158and the workpiece.

Still other spring arrangements are possible, including an arrangementhaving a plurality of spring elements 161-161 which extend from the rearmovable mounting member 163 to the fixed mounting member 165 in asomewhat basket-like arrangement. The spring elements 161-161 angleoutwardly between the two mounting members. FIG. 11 shows such anarrangement with four springs. More springs could be added. The plane ofeach spring 161 extends through the center of rotation of the assembly.A drive shaft, (not shown) at the center of rotation of the springassembly, will extend through opening 167 in fixed member 165 frommovable member 163.

All of the above alternative configurations have the characteristic thatthe spring member is less resistant to twisting motion than to bendingmotion, producing a rotational action of the drive shaft in response toa side-to-side driving force.

FIG. 4 shows an alternative application for the present invention. Thisapplication, which is an electric shaver 80, also includes a movable endpiece 81 with permanent magnets, driven by a magnetic driverarrangement, shown generally at 82. Extending from the movable end piece81 are the two angled leaf springs 84 and 86, which are fixedly securedat their other ends to a fixed end piece 88. A drive shaft 90 extendsrearwardly from the fixed end piece 81, parallel with the leaf springs84, 86. The drive shaft 90 is suspended from the movable end piece 81 bysupport elements 82 and extends beneath the driver assembly 82.

Fixedly attached to and extending radially outwardly from drive shaft 90are a plurality of elongated cutter blades 94, the outboard edges ofwhich are sharpened. The outboard edges mate against a curved shaverscreen element 96. In operation, the side-to-side action of the endpiece 81 results in a rotation of drive shaft 90, and in turn a rotationof the cutter blades 84 through a specific angle, providing the cuttingaction for the shaver.

This embodiment demonstrates that the position and arrangement of thedrive shaft on which the workpiece is mounted can take variousconfigurations relative to the motion conversion spring assembly. In allcases, however, a movable end piece is driven in a side-by-side fashion,with a spring assembly extending from the movable end piece to a fixedend piece, which produces a rotary action of the drive shaft which ismounted to the movable end piece and hence a rotary action of theworkpiece mounted thereon.

The present invention eliminates the need for any bearing elements orflex elements, primarily because there is little bending of the springelement. It is, however, a strong, reliable and effective structure.Furthermore, it is a resonant assembly, which can be matched to anoperating (driving) frequency of the appliance. For instance, in thetoothbrush embodiment, the desired operating frequency is approximately261 Hz. The spring assembly can be arranged and constructed so that itprovides rotational action at that frequency.

As indicated above, the toothbrush of FIG. 1, including the motionconversion assembly, is a “resonant” system. That is, the naturalresonant frequency of the mechanical elements of the system are designedto match the operating or drive frequency of the toothbrush, which inthe case of the present toothbrush is approximately 261 Hz. Such aresonant system is more efficient than a non-resonant system.

As discussed above, many approaches are known for ensuring that theresonant frequency of the spring system matches the operating frequencyof the appliance. In one approach, the configuration and arrangement ofthe mechanical system is sufficiently accurately controlled duringmanufacture to provide the desired frequency match without tuning.Manufacturing tolerances must, however, be quite tight (including usingmore precise parts), and a substantial number of appliances are usuallydiscarded because they do not meet the required strict manufacturingtolerances.

In another approach, various actions are taken to “tune” or modify thespring structure to produce the required frequency match with theoperating frequency of the appliance. The typical tuning process usuallytakes a substantial amount of time and expertise and is hence typicallynot cost-effective. Such tuning can be accomplished during manufactureor at times thereafter, even when the appliance is in the possession ofthe user.

In this aspect of the present invention, a method is disclosed fortuning a resonant spring system of a high volume product using leafsprings. The present method can be used to either increase or decrease(tuning up or tuning down) the resonant frequency of the spring assemblyto match the operating frequency.

In a first step of the process, measurement of amplitude of theworkpiece is made at a test frequency or vice versa. A fixed amplitudecan be used with a variable frequency or a fixed frequency can be usedwith a variable amplitude. Both techniques can be used for tuning.Tuning the spring assembly shifts the operating point of the springassembly on the operating characteristic curves to match the desiredoperating frequency or amplitude.

The point on the characteristic operating curve can be decreased (adecrease in frequency) by reducing the spring rate of the springassembly. Alternatively, the operating point can be increased (anincrease in frequency) by increasing the spring rate. While many tuningsystems increase or decrease the inertia of the system by increasing ordecreasing the weight of the spring system, this approach is often notvery accurate and requires a fairly high amount of weight reduction orincrease to produce the desired frequency matching effect. In thepresent invention, however, the focus is on increasing or decreasing thespring rate (instead of inertia) by changing the configuration of thespring assembly which, in the present case, are leaf springs.

In the process to reduce the spring rate, the individual leaf springs asmanufactured are cut, as shown in FIG. 5, to alter their configuration,which changes the spring rate of the leaf spring, while material can beadded to the springs to increase the spring rate. In the embodimentshown, cut 110 reduces the spring width, as shown at 100, the cut beingcentered generally at a point equidistant between the two ends 102, 104of the spring element 106, to minimize fatigue-generating stresses. Thedepth of the cut (across the width of the spring) is selected to producethe degree of tuning required. The variables of the cut include theoverall length of the cut, the configuration, i.e. the curvature of thecut, and the width of the cut. In the specific embodiment shown, the cut110 is curved in configuration. It should be understood, however, thatother cut configurations can be used. One alternative is a “football”shaped slot in the center of one or both springs. Variables for this cutinclude the location of the slot, the length and/or width of the slotand the curvature of the slot. The cut can be accomplished by a numberof techniques, including a laser, a grinder or a conventional cuttingtool. The manner in which the material is removed is not significant tothe invention.

After the cut has been made, another test is made to confirm that thefrequency of the structure is within accepted tolerance. If not,additional cuts can be made to bring the device into tolerance. Theresult of the tuning of the present system is that the manufacturingtolerance of such appliances need no longer be as severe as heretofore.This will reduce the number of rejects of the manufactured product. Inaddition, the tuning can be accomplished quickly and efficiently duringmanufacturing and the appliance requires no additional tuningthereafter.

Hence, first, a new motion conversion assembly from side-by-side torotary action for an appliance such as a toothbrush has been disclosed,using in one preferred embodiment leaf springs which are configured tobe less resistant to twisting than to bending, even though there is somebending by the spring assembly, in addition to the twisting, to producethe desired rotary action of a drive shaft on which the workpiece ismounted. High resistance to bending eliminates the need for bearings.The motion being converted is referred to as side-to-side, whichincludes a straight line movement as well as movement which includes aslight arc, as in the present embodiment. The workpiece motion isrotary, about the axis of the drive shaft on which the workpiece ismounted. Second, a method has been disclosed for tuning a springassembly system to change the resonant frequency of the system duringmanufacturing to conform to the desired operating frequency (drivingfrequency) of the system. This permits the use of less severemanufacturing tolerances.

Although a preferred embodiment of both the motion conversion system andmethod of tuning has been described for purposes of illustration, itshould be understood that various changes, modification andsubstitutions can be used in such embodiments without departing from thespirit of the invention which is defined by the claims which follow.

What is claimed:
 1. A method for tuning a leaf spring assembly used in aresonant driving system for a power toothbrush appliance having aworkpiece and a driving frequency, the method being a part of amanufacturing process for the toothbrush appliance, comprising the stepsof: determining the resonant frequency of the leaf spring assembly,following manufacture of the leaf spring assembly, with the workpiecemoving at a fixed amplitude, as part of a manufacturing process of thepower toothbrush appliance, wherein the leaf spring assembly has aconfiguration and wherein the determined resonant frequency is eitherabove or below the driving frequency of the toothbrush appliance;changing the configuration of the leaf spring assembly, in order toshift an operating point of the leaf spring assembly on its operatingcharacteristic curve, such that the resulting resonant frequency of theleaf spring assembly is closer to the driving frequency of thetoothbrush appliance than previously, if the determined resonantfrequency is not within a pre-selected tolerance relative to the drivingfrequency; determining again the resonant frequency of the leaf springassembly, with the workpiece moving at said fixed amplitude to determinewhether the resulting resonant frequency is within the pre-selectedtolerance; and repeating the steps of changing and determining againuntil the resonant frequency of the leaf spring assembly is within thepre-selected tolerance.
 2. A method of claim 1, including the step ofremoving a portion of the leaf spring assembly to decrease its resonantfrequency.
 3. A method of claim 1, including the step of adding materialto the leaf spring assembly to increase its resonant frequency.
 4. Amethod of claim 3, wherein the configuration of the removed portiondefines a gradual reduction of the spring assembly width over the lengthof the removed portion, with the maximum extent of the removed portionat approximately a midpoint of the removed portion, the removed portionbeing located at approximately midway between two ends of the leafspring assembly.
 5. A method of claim 4, wherein the removed portion iscurved between the respective ends thereof.
 6. A method of claim 1,wherein the leaf spring assembly comprises two leaf members positionedat a selected angle to each other, wherein the method includes the stepof removing a portion of each leaf spring.